Electric storage battery construction and method of manufacture

ABSTRACT

An electric storage battery and method of manufacture thereof characterized by a feedthrough pin which is internally directly physically and electrically connected to an inner end of a positive electrode substrate. A C-shaped mandrel extends around the pin and substrate end enabling the pin/mandrel to be used during the manufacturing process as an arbor to facilitate winding layers of a spiral jellyroll electrode assembly. The pin additionally extends from the battery case and in the final product constitutes one of the battery terminals with the battery case comprising the other terminal. Active material is removed from both sides of the outer end of the negative electrode in the jellyroll to allow room for adhesive tape to secure the jellyroll. The electrolyte is injected through the open end of the case after the endcap is welded to the negative electrode but before sealing the endcap to the case. The electrolyte is preferably injected through the C-shaped mandrel to facilitate and speed filling.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/348,665, filed Jan. 15, 2002.

FIELD OF THE INVENTION

[0002] This invention relates generally to electric storage batteriesand more particularly to a battery construction, and method ofmanufacture thereof, suitable for use in implantable medical devices.

BACKGROUND OF THE INVENTION

[0003] Rechargeable electric storage batteries are commerciallyavailable in a wide range of sizes for use in a variety of applications.As battery technology continues to improve, batteries find newapplications which impose increasingly stringent specifications relatingto physical size and performance. Thus, new technologies have yieldedsmaller and lighter weight batteries having longer storage lives andhigher energy output capabilities enabling them to be used in anincreasing range of applications, including medical applications, where,for example, the battery can be used in a medical device which isimplanted in a patient's body. Such medical devices can be used tomonitor and/or treat various medical conditions.

[0004] Batteries for implantable medical devices are subject to verydemanding requirements, including long useful life, high power output,low self-discharge rates, compact size, high reliability over a longtime period, compatibility with the patient's internal body chemistry,etc. Although various battery chemistries have been tried, lithium iontechnology is generally accepted as the preferred chemistry for medicalimplant applications.

[0005] Such electric storage batteries are generally comprised of atubular metal case enveloping an interior cavity which contains anelectrode assembly surrounded by a suitable electrolyte. The electrodeassembly generally comprises a plurality of positive electrode, negativeelectrode, and separator layers which are typically stacked and/orspirally wound to form a jellyroll. The positive electrode is generallyformed of a metal substrate having positive active material coated onboth faces of the substrate. Similarly, the negative electrode is formedof a metal substrate having negative active material coated on bothfaces of the substrate. In forming an electrode assembly, separatorlayers are interleaved between the positive and negative electrodelayers to provide electrical isolation.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to an electric storage batteryincorporating one or more aspects described herein for enhancing batteryreliability while minimizing battery size. In addition, the invention isdirected to a method for efficiently manufacturing the battery at arelatively low cost.

[0007] In accordance with a first significant aspect of the invention, afeedthrough pin is provided which is directly physically andelectrically connected to the inner end of an electrode substrate (e.g.,positive), as by welding. The pin is used during the manufacturingprocess as an arbor to facilitate winding the layers to form anelectrode assembly jellyroll. Additionally, in the fully manufacturedbattery, the pin extends through a battery case endcap and functions asone of the battery terminals. The battery case itself generallyfunctions as the other battery terminal.

[0008] More particularly, in accordance with an exemplary preferredembodiment, the inner end of the positive electrode substrate is spotwelded to the feedthrough pin to form an electrical connection. Thesubstrate, e.g., aluminum, can be very thin, e.g., 0.02 mm, making itdifficult to form a strong mechanical connection to the pin, which ispreferably constructed of a low electrical resistance, highly corrosionresistant material, e.g., platinum iridium, and can have a diameter onthe order of 0.40 mm. In order to mechanically reinforce the pin andsecure the pin/substrate connection, a slotted C-shaped mandrel isprovided. The mandrel is formed of electrically conductive material,e.g., titanium-6Al-4V, and is fitted around the pin, overlaying thepin/substrate connection. The mandrel is then preferably welded to boththe pin and substrate. The mandrel slot defines a keyway foraccommodating a drive key which can be driven to rotate the mandrel andpin to wind the electrode assembly layers to form the spiral jellyroll.

[0009] In accordance with a further significant aspect of the invention,the outer layer of the jellyroll is particularly configured to minimizethe size, i.e., outer radius dimension, of the jellyroll. Moreparticularly, in the exemplary preferred embodiment, the active materialis removed from both faces of the negative electrode substrate adjacentits outer end. The thickness of each active material coat can be about0.04 mm and the thickness of the negative substrate can be about 0.005mm. By baring the outer end of the negative electrode substrate, it canbe adhered directly, e.g., by an appropriate adhesive tape, to the nextinner layer to close the jellyroll to while minimizing the roll outerradius dimension.

[0010] A battery case in accordance with the invention is comprised of atubular case body having open first and second ends. The feedthrough pinpreferably carries a first endcap physically secured to, butelectrically insulated from, the pin. This first endcap is preferablysecured to the case body, as by laser welding, to close the open firstend and form a leak free seal. With the jellyroll mounted in the caseand the first endcap sealed, the interior cavity can thereafter befilled with electrolyte from the open second end.

[0011] In accordance with a still further aspect of the invention, thejellyroll assembly is formed with a flexible electrically conductive tabextending from the negative electrode substrate for electricalconnection to the battery case. In accordance with a preferredembodiment, the tab is welded to a second endcap which is in turn weldedto the case. The tab is sufficiently flexible to enable the secondendcap to close the case body second end after the interior cavity isfilled with electrolyte via the open second end. In accordance with anexemplary preferred embodiment, the tab is welded to the inner face ofthe second endcap such that when the jellyroll is placed in the body,the tab locates the second endcap proximate to the body withoutobstructing the open second end. After electrolyte filling, the casebody is sealed by bending the tab to position the second endcap acrossthe body second end and then laser welding the endcap to the case body.

BRIEF DESCRIPTION OF THE FIGURES

[0012]FIG. 1 is a side view of a feedthrough pin subassembly inaccordance with the invention;

[0013]FIG. 2 is a longitudinal sectional view through the subassembly ofFIG. 1;

[0014]FIG. 3 is a plan view of a positive electrode strip utilized inthe exemplary preferred electrode assembly in accordance with theinvention;

[0015]FIG. 4 is a side view of the positive electrode strip of FIG. 3;

[0016]FIG. 5 is an enlarged sectional view of the area A of FIG. 4showing the inner end of the positive electrode strip of FIGS. 3 and 4;

[0017]FIG. 6 is an isometric view showing the bared inner end of thepositive electrode substrate spot welded to the feedthrough pin andconfigured to receive a C-shaped mandrel thereon;

[0018]FIG. 7 is an end view showing the C-shaped mandrel being crimpedto the pin and electrode;

[0019]FIG. 8 is an end view showing the C-shaped mandrel mounted on thepin and capturing the positive electrode substrate therebetween;

[0020]FIG. 9 is an isometric view depicting a drive key accommodated inthe slot of the C-shaped mandrel;

[0021]FIG. 10 is a plan view showing the drive key coupled to a drivemotor for rotating the C-shaped mandrel;

[0022]FIG. 11 is a schematic end view depicting how rotation of theC-shaped mandrel and pin can wind positive electrode, negativeelectrode, and separator strips to form a spiral jellyroll electrodeassembly;

[0023]FIG. 12 is a plan view of a negative electrode strip utilized inthe exemplary preferred electrode assembly in accordance with theinvention;

[0024]FIG. 13 is a side view of the negative electrode strip of FIG. 12;

[0025]FIG. 14 is an enlarged sectional view of the area A of FIG. 13showing the inner end of the negative electrode strip of FIGS. 12 and13;

[0026]FIG. 15 is an enlarged sectional view of the area B of FIG. 13showing the outer end of the negative electrode strip of FIGS. 11 and12;

[0027]FIG. 16 is an isometric view showing that the layers of a spirallywound electrode assembly, i.e., jellyroll;

[0028]FIG. 17 is a plan view of the negative electrode strip showing theattachment of a flexible electrically conductive tab to the bared outerend of the negative electrode substrate;

[0029]FIG. 18 is an enlarged sectional view showing how the outer turnof the negative electrode strip is taped to the next inner layer toclose the jellyroll to minimize its outer radius dimension;

[0030]FIG. 19 is an isometric view depicting the jellyroll electrodeassembly being inserted into a cylindrical battery case body;

[0031]FIG. 20 is an isometric view showing a battery case body with thenegative electrode tab extending from the open case body;

[0032]FIG. 21 is an isometric view showing how the negative electrodetab is mechanically and electrically connected to an endcap for sealingthe case body second end;

[0033]FIG. 22 is a side view showing how the negative electrode tabholds the second endcap proximate to the case body second end withoutobstructing the open second end;

[0034]FIG. 23 is a front view showing the weld position and therelationship between the various components; and

[0035]FIG. 24 is an enlarged sectional view of the second end of thebattery case showing the endcap in sealed position.

DETAILED DESCRIPTION

[0036] Attention is initially directed to FIGS. 1 and 2 which illustratea preferred feedthrough pin subassembly 10 utilized in accordance withthe present invention. The subassembly 10 is comprised of an elongatepin 12, preferably formed of a solid electrically conductive material,having low electrical resistance and high corrosion resistance such asplatinum iridium, preferably 90 Pt/10 Ir. The pin 12 extends through,and is hermetically sealed to a header 14. The header 14 is comprised ofdielectric disks, e.g., ceramic, 16 and 18 which sandwich a glass hollowcylinder 20 therebetween. The glass hollow cylinder is hermeticallysealed to the pin 12. The outer surface of the glass hollow cylinder 20is sealed to the inner surface of an electrically conductive hollowmember 22, e.g., titanium-6Al-4V. As will be seen hereinafter, theconductive hollow material 22 functions as a battery case endcap in thefinal product to be described hereinafter.

[0037] Attention is now directed to FIGS. 3, 4, and 5 which illustrate apreferred positive electrode strip 30 which is utilized in thefabrication of a preferred spirally wound jellyroll electrode assemblyin accordance with the present invention. The positive electrode strip30 is comprised of a metal substrate 32 formed, for example, ofaluminum. Positive electrode active material 34, 36 is deposited,respectively on the upper and lower faces 38 and 40 of the substrate 32.Note in FIGS. 3, 4, and 5 that the right end of the substrate 32 isbare, i.e. devoid of positive active material on both the upper andlower faces 38, 40.

[0038] It is to be pointed out that exemplary dimensions are depicted inFIGS. 1-5 and other figures herein. These exemplary dimensions areprovided primarily to convey an order of magnitude to the reader tofacilitate an understanding of the text and drawings. Although theindicated dimensions accurately reflect one exemplary embodiment of theinvention, it should be appreciated that the invention can be practicedutilizing components having significantly different dimensions.

[0039]FIG. 6 depicts an early process step for manufacturing a batteryin accordance with the invention utilizing the pin subassembly 10 (FIGS.1, 2) and the positive electrode strip 30 (FIGS. 3-5). A topsideelectrode insulator (not shown), which may comprise a thin disk ofDuPont Kapton® polyimide film, is slipped onto the pin 12 adjacent theheader 14. In accordance with the present invention, the bare end of theelectrode strip substrate 32 is electrically connected to the pin 12preferably by resistance spot welding, shown at 44. Alternatively,substrate 32 may be ultrasonically welded to the pin 12. The thinness,e.g. point 0.02 mm of the substrate 32, makes it very difficult to forma strong mechanical connection between the substrate and the pin 12.Accordingly, in accordance with a significant aspect of the presentinvention, an elongate C-shaped mandrel 48 is provided to mechanicallyreinforce the pin 12 and secure the substrate 32 thereto.

[0040] The mandrel 48 preferably comprises an elongate titanium ortitanium alloy such as Ti-6Al-4V tube 50 having a longitudinal slot 52extending along the length thereof. The arrow 54 in FIG. 6 depicts howthe mandrel 48 is slid over the pin 12 and substrate 32, preferablyoverlaying the line of spot welds 44. The mandrel 48, pin 12, andsubstrate 32 are then preferably welded together, such as by resistancespot welding or by ultrasonic welding. Alternatively, the mandrel 48 maybe crimped onto the pin 12 at least partially closing the “C” to createa strong mechanical connection. In the case of forming only a mechanicalconnection and not necessarily a gas-tight electrical connection betweenthe mandrel 48 and the pin and substrate, the mandrel material ispreferably made of a material that will not lead to electrolysis. Whenused with electrolytes that tend to contain hydrofluoric acid, themandrel is preferably made of 304, 314, or 316 stainless steels oraluminum or an alloy thereof chosen for its compatibility with the othermaterials. FIG. 7 is an end view showing the step of crimping themandrel 48 to the pin 12 and substrate 32. Supporting die 126 is used tosupport the mandrel 48 and crimping dies 124 and 125 are used to deformthe edges of the mandrel 48 to bring them closer together andmechanically connect the mandrel 48 to the pin 12 and substrate 32. Bycrimping in the direction of arrows 127 and 128, a strong connection isformed without damaging the thin electrode or disturbing the electricalconnection between the pin and the electrode.

[0041]FIG. 8 is an end view showing the slotted mandrel 48 on the pin 12with the substrate 32 extending tangentially to the pin 12 andterminating adjacent the interior surface of the mandrel tube 50. Thetube 50 is preferably sufficiently long so as to extend beyond the freeend of the pin 12. As depicted in FIG. 9, this enables a drive key 56 toextend into the mandrel slot 52.

[0042]FIG. 10 schematically depicts a drive motor 60 for driving thedrive key 56 extending into mandrel slot 52. With the pin subassemblyheader 14 supported for rotation (not shown), energization of the motor60 will orbit the key drive 56 to rotate the mandrel 48 and subassembly10 around their common longitudinal axes. The rotation of the mandrel 48and subassembly 10 is employed to form a jellyroll electrode assembly inaccordance with the present invention.

[0043] More particularly, FIG. 11 depicts how a jellyroll electrodeassembly is formed in accordance with the present invention. The bareend of the substrate 32 of the positive electrode strip 30 iselectrically connected to the pin 12 as previously described. Theconductive mandrel 48 contains the pin 12 and bare substrate end, beingwelded to both as previously described. A strip of insulating separatormaterial 64 extending from opposite directions is introduced between themandrel 48 and positive electrode substrate 32, as shown. A negativeelectrode strip 70 is then introduced between the portions of theseparator material extending outwardly from mandrel 48.

[0044] The preferred exemplary negative electrode strip 70 is depictedin FIGS. 12-15. The negative electrode strip 70 is comprised of asubstrate 72, e.g. titanium, having negative active material formed onrespective faces of the substrate. More particularly, note in FIG. 14that negative active material 74 is deposited on the substrate uppersurface 76 and negative active material 78 is deposited on the substratelower surface 80. FIG. 14 depicts the preferred configuration of theinner end 82 of the negative electrode strip 70 shown at the left ofFIGS. 12 and 13. FIG. 15 depicts the configuration of the outer end 83of the negative electrode strip 70 shown at the right side of FIGS. 12and 13.

[0045] Note in FIG. 14 that one face of the substrate inner end 82 isbared. This configuration can also be noted in FIG. 11 which shows howthe negative substrate inner end 82 is inserted between turns of theseparator strip 64. After the strip 70 has been inserted as depicted inFIG. 11, the aforementioned drive motor 60 is energized to rotate pin 12and mandrel 48, via drive key 56, in a counterclockwise direction, asviewed in FIG. 11. Rotation of pin 12 and mandrel 48 functions to windpositive electrode strip 30, separator strip 64, and negative electrodestrip 70, into the spiral jellyroll assembly 84, depicted in FIG. 16.The assembly 84 is comprised of multiple layers of strip material sothat a cross section through the assembly 84 would reveal a sequence oflayers in the form pos/sep/neg/sep/pos/sep/neg/ . . . , etc.

[0046]FIG. 15 depicts a preferred configuration of the outer end 83 ofthe negative electrode strip 70. Note that the outer end 88 of thesubstrate 72 is bared on both its top and bottom faces. Additionally, asshown in FIG. 17, a flexible metal tab 90 is welded crosswise to thesubstrate 72 so as to extend beyond edge 92. More particularly, notethat portion 94 of tab 90 is cantilevered beyond edge 92 of negativeelectrode strip 70. This tab portion, as will be described hereinafter,is utilized to mechanically and electrically connect to an endcap forclosing a battery case.

[0047] Attention is now called to FIG. 18, which illustrates a preferredtechnique for closing the jellyroll assembly 84. That is, the bared end88 of the negative electrode substrate 72 extending beyond the negativeactive material coat 78 is draped over the next inner layer of thejellyroll assembly 84. The end 88 can then be secured to the next innerlayer, e.g., by appropriate adhesive tape 96. One such suitable adhesivetape is DuPont Kapton® polyimide tape. It is important to note that theouter end configuration 88 of the negative electrode strip 70 enablesthe outer radius dimension of the jellyroll assembly 84 to be minimizedas shown in FIG. 18. More particularly, by baring the substrate 72beyond the active material 78, the tape 96 is able to secure thesubstrate end without adding any radial dimension to the jellyrollassembly. In other words, if the outer end of the substrate were notsufficiently bared, then the tape 96 would need to extend over theactive material and thus add to the outer radius dimension of thejellyroll 84. Furthermore, the bare substrate 72 is more flexible thanthe substrate coated with active material 78 and conforms more readilyto the jellyroll assembly 84, making it easier to adhere it to thesurface of the jellyroll. These space savings, although seemingly small,can be clinically important in certain medical applications. It shouldbe noted that the electrode need only be bared at an end portion longenough to accommodate the tape 96, as shown in FIG. 18. Because theuncoated substrate does not function as an electrode, it would wastespace in the battery to bare any more than necessary to accommodate thetape. In a preferred embodiment, the length of uncoated substrate isbetween 1 and 8 mm, and more preferably about 2 mm.

[0048]FIG. 19 depicts the completed jellyroll assembly 84 and shows thecantilevered tab portion 94 prior to insertion into a battery case body100. The case body 100 is depicted as comprising a cylindrical metaltube 101 having an open first end 104 and open second end 106. Arrow 107represents how the jellyroll assembly 84 is inserted into thecylindrical tube 101. FIG. 20 depicts the jellyroll assembly 84 withinthe tube 101 with the cantilevered negative electrode tab 94 extendingfrom the case open second end 106. The case open first end 104 is closedby the aforementioned header 14 of the pin subassembly 10 shown in FIGS.1 and 2. More particularly, note that the metal hollow member 22 isconfigured to define a reduced diameter portion 108 and shoulder 110.The reduced diameter portion 108 is dimensioned to fit into the open end104 of the cylindrical tube 101 essentially contiguous with the tube'sinner wall surface. The shoulder 110 of the hollow member 22 engages theend of the case tube 101. This enables the surfaces of the reduceddiameter portion 108 and shoulder 110 to be laser welded to the end ofthe case 100 to achieve a hermetic seal.

[0049] Attention is now directed to FIGS. 21-24, which depict the tab 94extending from the second open end 106 of the case tube 101. Note thatthe tab 94 extends longitudinally from the body close to the casetube-adjacent to tube's inner wall surface. In accordance with apreferred embodiment of the invention, the tab 94 is welded at 110 tothe inner face 112 of a circular second endcap 114. In accordance with apreferred embodiment, the tab 94 is sufficiently long to locate the weld110 beyond the center point of the circular endcap 114. Moreparticularly, note in FIGS. 20-23 that by locating the weld 110displaced from the center of the cap 114, the tab 94 can convenientlysupport the endcap 114 in a vertical orientation as depicted in FIG. 21misaligned with respect to the open end 106. This end cap positionapproximately perpendicular to the end 122 of the case 100 is a firstbias position wherein the end cap advantageously tends to remain in thatorientation with the case end open prior to filling. To further describethe relationship between the weld location and the various components,FIG. 23 shows a front view with various dimensions. L represents thelength from the weld 110 to the top of the case 101 as measured parallelto the edge of the case. R is the radius of the end cap 114. For thepreferred geometry, L≦2R. Weld 110 is preferably made above the centerpoint 111 of the end cap 114. Preferably, the end cap 114 overlaps thecase 101 by approximately R/2. By configuring the tab 94 and weld 110 asindicated, the endcap 114 can be supported so that it does not obstructthe open end 106, thereby facilitating electrolyte filling of the caseinterior cavity via open end 106. A filling needle or nozzle can beplaced through open end 106 to fill the case. This obviates the need fora separate electrolyte fill port, thereby reducing the number ofcomponents and number of seals to be made, thus reducing cost andimproving reliability. Furthermore, for small medical batteries, the endcaps would be very small to have fill ports therein. In a preferredembodiment in which the case wall is very thin, for example, 0.002inches, providing a fill port in the side wall of the case would beimpractical. Even in the case of larger devices where space is lesscritical and the wall is more substantial, providing a fill port in theside of the case would mean the electrolyte would have a very long pathlength to wet the jellyroll. Note that while the case could be filledwith electrolyte prior to welding tab 94 to endcap 114, it would bedifficult and messy to do so. Therefore, it is advantageous to configurethe tab 94 and weld 110 as described to allow the weld to be made priorto filling.

[0050] Preferably before filling, a bottomside electrode insulator (notshown), which may comprise a thin disk of DuPont Kapton® polyimide film,is installed into the case between the rolled electrode assembly and thestill open end of the battery case.

[0051] In a preferred filing method, there is a channel of air betweenthe pin and the crimped or welded C-shaped mandrel, which is used as aconduit for quickly delivering the electrolyte to the far end of thebattery and to the inside edges of the electrodes within the jellyroll.Filling from the far end of the battery prevents pockets of air frombeing trapped, which could form a barrier to further filling. Thisfacilitates and speeds the filling process, ensuring that electrolytewets the entire battery.

[0052] Thereafter, the flexible tab 94 can be bent to the configurationdepicted in FIG. 24. Note that the endcap 114 is configured similarly toheader hollow member 22 and includes a reduced diameter portion 118 anda shoulder 120. The reduced diameter portion snugly fits against theinner surface of the wall of tube 101 with the endcap shoulder 120bearing against the end 122 of the cylindrical case 100. The relativelylong length of the tab 94 extending beyond the center point of theendcap surface 112 minimizes any axial force which might be exerted bythe tab portion 94 tending to longitudinally displace the endcap 114.The end cap position covering the end 122 of the case 100 is a secondbias position wherein the end cap advantageously tends to remain in thatorientation prior to welding. With the endcap in place, it can then bereadily welded to the case wall 101 to hermetically seal the battery.With tab 90 welded to negative substrate 72 and with the negativeelectrode strip 70 as the outermost layer of the jellyroll, the endcap114 becomes negative. In turn, welding the endcap 114 to the case 100renders the case negative.

[0053] From the foregoing, it should now be appreciated that an electricstorage battery construction and method of manufacture have beendescribed herein particularly suited for manufacturing very small,highly reliable batteries suitable for use in implantable medicaldevices. Although a particular preferred embodiment has been describedherein and exemplary dimensions have been mentioned, it should beunderstood that many variations and modifications may occur to thoseskilled in the art falling within the spirit of the invention and theintended scope of the appended claims.

We claim:
 1. An electrode assembly for use in an electric storagebattery, said electrode assembly comprising: a first polarity electrodestrip comprising an elongate first substrate having first and secondfaces each carrying a coat of active material; a second polarityelectrode strip comprising an elongate second substrate having first andsecond faces each carrying a coat of active material; said first andsecond electrode strips being superposed and wound together around acommon axis to form a spiral roll having successive alternating layersof said first and second electrode strips, said layers including aninner first polarity electrode layer of minimum radius proximate to aninner end of said first substrate and an outer second polarity electrodelayer of maximum radius proximate to an outer end of second substrate;said spiral roll including layers of separator material interleavedbetween layers of said first and second electrode strips; and whereinsaid second substrate outer end is devoid of active material on bothfaces thereof whereby the outer radius of said roll can be minimized byadhering said second substrate outer end to an adjacent roll layer. 2.The assembly of claim 1 wherein the thickness of each coat of activematerial on said second substrate is greater than the thickness of saidsecond substrate.
 3. The assembly of claim 1 further including: adhesivetape adhering said second substrate outer end to said adjacent rolllayer.
 4. The assembly of claim 1 wherein the combined thickness of thecoats of active material on said second substrate is greater than thethickness of said adhesive tape.
 5. The assembly of claim 1 wherein saidsecond substrate outer end is devoid of active material on both facesthereof along a length less than a full turn of said spiral roll.
 6. Theassembly of claim 1 wherein said second substrate outer end is devoid ofactive material on both faces thereof along a length of between 1 and 8mm.
 7. The assembly of claim 1 wherein said second substrate outer endis devoid of active material on both faces thereof along a length ofabout 2 mm.
 8. The assembly of claim 1 wherein said second polarityelectrode strip is negative.
 9. An electric storage battery comprising:an electrically conductive case; the electrode assembly of claim 1housed in said case; wherein said case is electrically coupled to saidsecond polarity electrode strip.
 10. The electric storage battery ofclaim 9 wherein said second polarity electrode strip is negative andwherein said case is negative.
 11. An electrode assembly for use in anelectric storage battery, said electrode assembly made by the steps of:providing a first polarity electrode strip comprising an elongate firstsubstrate having first and second faces each carrying a coat of activematerial; providing a second polarity electrode strip comprising anelongate second substrate having first and second faces each carrying acoat of active material, wherein an outer end is devoid of activematerial on both faces thereof; providing separator material;interleaving said first and second electrode strips with said separatormaterial; winding said interleaved electrode strips and separatormaterial together around a common axis to form a spiral roll havingsuccessive alternating layers of said first electrode strip, saidseparator material, said second electrode strip, and said separatormaterial, said layers including an inner first polarity electrode layerof minimum radius proximate to an inner end of said first substrate andan outer second polarity electrode layer of maximum radius proximate tothe outer end of second substrate, whereby the outer end is devoid ofactive material.
 12. The assembly of claim 11 wherein said step ofproviding a second polarity electrode strip comprises providing a secondpolarity electrode strip wherein the thickness of each coat of activematerial on the second substrate is greater than the thickness of thesecond substrate.
 13. The assembly of claim 11 wherein said stepsfurther comprise a step of: adhering the second substrate outer end tothe adjacent roll layer with adhesive tape.
 14. The assembly of claim 13wherein said step of providing a second polarity electrode stripcomprises providing a second polarity electrode strip wherein thecombined thicknesses of the coats of active material on the secondsubstrate is greater than the thickness of the adhesive tape.
 15. Theassembly of claim 11 wherein said second substrate outer end is devoidof active material on both faces thereof along a length less than a fullturn of said spiral roll.
 16. The assembly of claim 11 where said secondsubstrate outer end is devoid of active material on both faces thereofalong a length of between 1 and 8 mm.
 17. The assembly of claim 11wherein said second substrate outer end is devoid of active material onboth faces thereof along a length of about 2 mm.
 18. The assembly ofclaim 11 wherein said second polarity electrode strip is negative. 19.An method for making an electrode assembly for use in an electricstorage battery, comprising the steps of; providing a first polarityelectrode strip comprising an elongate first substrate having first andsecond faces each carrying a coat of active material; providing a secondpolarity electrode strip comprising an elongate second substrate havingfirst and second faces each carrying a coat of active material, whereinan outer end is devoid of active material on both faces thereof;providing separator material; interleaving said first and secondelectrode strips with said separator material; winding said interleavedelectrode strips and separator material together around a common axis toform a spiral roll having successive alternating layers of said firstelectrode strip, said separator material, said second electrode strip,and said separator material, said layers including an inner firstpolarity electrode layer of minimum radius proximate to an inner end ofsaid first substrate and an outer second polarity electrode layer ofmaximum radius proximate to the outer end of second substrate, wherebythe outer end is devoid of active material.
 20. The method of claim 19wherein said step of providing a second polarity electrode stripcomprises providing a second polarity electrode strip wherein thethickness of each coat of active material on the second substrate isgreater than the thickness of the second substrate.
 21. The method ofclaim 19 wherein said steps further comprise a step of: adhering thesecond substrate outer end to the adjacent roll layer with adhesivetape.
 22. The method of claim 19 wherein said step of providing a secondpolarity electrode strip comprises providing a second polarity electrodestrip wherein the combined thicknesses of the coats of active materialon the second substrate is greater than the thickness of the adhesivetape.